Air pollutants are major contributors to climate change. This connection is well known to scientists, although it has not yet permeated environmental policy. Figure 19 shows global climate forcing for the year 2000, relative to 1850, caused by different observed perturbations to the Earth system. Climate forcing from a given perturbation is defined as the mean resulting imbalance between energy input and energy output per unit time and unit area of Earth's surface (watts per square meter or W/m2), with all else remaining constant, including temperature. A positive radiative forcing means a decrease in energy output and hence a warming, shown as red bars in Fig. 19. Negative radiative forcing, shown as blue bars in Fig. 19, means a decrease in energy input and hence a cooling. (For more on climate forcing see Unit 12, "Earth's Changing Climate.")

Figure 19 shows that a large number of perturbation agents have forced the climate since 1850. CO2 is the single most important agent, but many other agents are also important and together exert greater influence than CO2.

Among the major greenhouse gases in Figure 19 are methane and tropospheric ozone, which are both of concern for air quality. Light absorption by black carbon aerosol particles also has a significant warming effect. Taken together these three agents produce more radiative forcing than CO2. Reductions in these air pollutants thus would reap considerable benefit for climate change.

However, air pollutants can also have a cooling effect that compensates for greenhouse warming. This factor can be seen in Figure 19 from the negative radiative forcings due to non light-absorbing sulfate and organic aerosols originating from fossil fuel combustion. Scattering by these aerosols is estimated by the Intergovernmental Panel on Climate Change (IPCC) to have a direct radiative forcing of -1.3 W/m2, although this figure is highly uncertain. Indirect radiative forcing from increased cloud reflectivity due to anthropogenic aerosols is even more uncertain but could be as large as -1 W/m2.

Scattering aerosols have thus masked a significant fraction of the warming imposed by increasing concentrations of greenhouse gases over the past two centuries. Aerosol and acid rain control policies, though undeniably urgent to protect public health and ecosystems, will reduce this masking effect and expose us to more greenhouse warming.

Influence also runs the other way. Global climate change has the potential to magnify air pollution problems by raising Earth's temperature (contributing to tropospheric ozone formation) and increasing the frequency of stagnation events. Climate change is also expected to cause more forest fires and dust storms, which can cause severe air quality problems (Fig. 20).

The link between air pollution and climate change argues for developing environmental policies that will yield benefits in both areas. For example, researchers at Harvard University, Argonne National Laboratory, and the Environmental Protection Agency estimated in 2002 that reducing anthropogenic methane emissions by 50 percent would not only reduce greenhouse warming but also nearly halve the number of high-ozone events in the United States. Moreover, since methane contributes to background ozone levels worldwide, this approach would reduce ozone concentrations globally. In contrast, reducing NOx emissions—the main U.S. strategy for combating ozone—produces more localized reductions to ozone (footnote 6).

Finally, let us draw the distinction between stratospheric ozone depletion and climate change, since these two problems are often confused in the popular press. As summarized in Table 2, the causes, processes, and impacts of these two global perturbations to the Earth system are completely different, but they have some links. On the one hand, colder stratospheric temperatures due to increasing greenhouse gases intensify polar ozone loss by promoting PSC formation, as discussed in Section 9. On the other hand, CFCs are major greenhouse gases, and stratospheric ozone depletion exerts a slight cooling effect on the Earth's surface.